In the production of nitrogen gas by the cryogenic rectification of feed air, the refrigeration necessary to drive the separation is generally supplied by the turboexpansion of one or more process streams, such as a portion of the feed air. Such a turboexpansion system is effective but very energy intensive. Often the production of liquid nitrogen is desired in addition to the nitrogen gas. Such a system imposes a significantly higher refrigeration load on the overall cryogenic air separation plant because a significant amount of refrigeration exits the plant with the liquid nitrogen. This problem increases as the relative amount of nitrogen recovered as liquid increases. A system which can produce both nitrogen gas and liquid nitrogen using less energy than is heretofore required would be highly desirable.
Accordingly, it is an object to provide a cryogenic air separation method for the production of both nitrogen gas and liquid nitrogen which enables the production of both products with reduced energy usage compared with known methods.